Postnatal lung growth involves remodeling of the structure seen at birth as new alveoli are formed. To determine the role of collagen degradation in this process, in particular of the basement membrane component, we studied the synthesis of total collagen and the degradation of collagen types I or IV in a series of rats from birth to 29 days of age. During the period of rapid cell proliferation to day 11, the collagen level per mg lung did not change though the rate of synthesis increased. Up to 40% of new collagen was rapidly degraded. At the end of the growth phase, the interstitium became thinner and less cellular. Collagen synthesis slowly decreased as the total collagen content increased in the lung, and less than 20% of newly synthesized collagen was degraded. Type I collagenase activity was highest during the cell proliferation phase, though less than 20% was due to active enzyme. In contrast, type IV collagen breakdown, also maximal in the first 11 days, was almost all due to enzymes present in the active form. The results demonstrate that rapid degradation of collagen, particularly the type IV form present in basement membranes, occurs during the early phase of postnatal lung growth.